Elimination of lead embrittlement in steel

ABSTRACT

THIS INVENTION IS ADDRESSED TO THE ELIMINATION OF LEAD EMBRITTLEMENT IN STEELS IN WHICH AT LEAST ONE RARE EARTH METAL IS ADDED TO A STEEL TO PRODUCE HAVING IMPROVED MECHANICAL PROPERTIES AT ELEVATED TEMPERATURES.

April 10, 1973 N. N. BREYER E AL 3,726,669

ELIMINATION OF LEAD EMBRITTLEMENT IN STEEL Filed March 5. 1971 w F d fl m M w Z d w w w w d ..6 A d 0A 9 6 E G v z w n Y F m w m D w w ARFEQ E $533 Heaf 5 Hem A ebo n I I I I 0 M 6 0 W 0 0 m 5 3 2 Xv EQTE 2953mm w w mm d T519 a M M w m 1m N MMM /M United States Patent 3,726,669 ELIMINATION OF LEAD EMBRITTLEMENT IN STEEL Norman N. Breyer, Highland Park, and Sheldon Mostovoy, Homewood, Ill., assignors to La Salle Steel Company, Hammond, Ind.

Filed Mar. 3, 1971, Ser. No. 120,463 int. Cl. C22c 39/54 US. Cl. 75-129 11 Claims ABSTRACT OF THE DISCLOSURE This invention is addressed to the elimination of lead embrittlement in steels in which at least one rare earth metal is added to a steel to produce a steel having improved mechanical properties at elevated temperatures.

The invention herein described was made in the course of, or under a contract or subcontract thereunder with the Department of the Army.

This invention relates to steels, and more particularly to a method for reducing lead embrittlement in alloy steels.

Approximately thirty years ago, the production of steel containing lead as an additive to improve machinability of the steel was begun in the United States. Initially, such uses of lead were limited to lowcarbon, free machining grades of steels, and satisfactory production was achieved after dil'ficulties stemming from non-homogeneous distribution of lead and non-uniform size of lead particles in the steel were overcome. Leaded steels were thus generally widely accepted and resulted in substantial economic advantages in terms of improved machinability.

In more recent years, lead has been added to higher carbon, alloy steels, again for the purpose of improving machinability. The generalization that the mechanical properties of leaded steels were not substantially different from the mechanical properties of corresponding nonleaded steels was expected to hold true, and in general did in fact hold true. Then, however, a number of difiiculties were encountered with leaded, high strength alloy steel which was processed or used under high temperature conditions. In fact, near catastrophic failures were experienced during high temperature treatment of leaded, alloy steels.

Such failures prompted considerable research as to why leaded alloy steels were subject to failure at high temperatures. This research was not aided by the literature in the art since it was the early consensus that ditferences in the room-temperature mechanical properties of leaded and non-leaded steels were not statistically significant and that the only effect of lead on steel is to improve its machinability [Dolan and Price: Properties and Machinability of a Leaded Steel, Metals and Alloys, 20 (January 1940)].

Although continuing research found the effect of lead on mechanical properties of steel to be comparatively minor, the disparity between the mechanical properties of leaded alloy steels reported in the literature, and the service failures traced to change over from a non-leaded steel to a leaded steel, caused Mostovoy and Breyer to examine the elevated temperature tensile properties of heat treated alloy steels, The Effect of Lead on the Mechanical Properties of 4145 Steel, Transactions of the American Society for Metal, 61, 219-232 (June 1968).

Mostovoy and Breyer investigated the properties of a 4145 steel heat treated to room temperature ultimate tensile strengths ranging from 120 ksi. to 240 ksi. in 20 ksi. increments, and found severe embrittlement in the test temperature range of 400 to 900 F. At strength levels of up to 160 ksi., the ductility of leaded steel was about 40% of that of the same steel in a non-leaded condi- 3,725,559 Patented Apr. 10, 1973 tion, that is without lead, at about 700 F. At higher strength levels, the embrittlement was most severe with essentially no ductility being exhibited by the leaded steel at temperatures from 620 to 900 F.

The research carried out by Mostovoy and Breyer can best be understood by reference to FIG. 1 of the drawing, which is a graph of percent reduction in area versus temperature. As is Well known to those skilled in the art, when a metal tensile test specimen or a rod is subjected to high tensile stresses beyond the elastic limit, the specimen is elongated with a resultant decrease in the diameter of the specimen, and consequently the crosssectional area of the specimen. As high tensile stresses are increased in the specimen, it breaks at the point at which the diameter or the cross-sectional area of the specimen is the smallest. Thus, the decrease in the cross-sectional area of the specimen after elongation at the breaking point is a relative measure of the ductility or brittleness of the metal, and it is this property which is plotted in FIG. 1.

As can be seen in FIG. 1, the percent reduction in area for nonleaded steels increases at temperatures above 400 R, which means that the ductility increases with increasing test temperature. This behavior is considered normal for metals; the ductility increases with increasing test temperature. However, in the case of leaded steel as also shown in this figure, the percent reduction in area decreases rapidly at temperatures above 400 F. and does not recover until a temperature of the order of 900 F. is reached. Therefore, the embrittlement is very severe in this temperature range for leaded steel as pointed out above. Failures during processing or use of leaded steel parts subjected to stress in this temperature range have been encountered.

Mostovoy and Breyer initially hypothesized that the embrittlement was due to liquid metal embrittlement of the steel by the lead. However, the requirement that molten lead be present is inconsistent with the two facts discovered in that (l) the loss of ductility was found at least 200 F. below the melting point of lead (612 F.) (2) the ductility recovers at temperature above 900 F. Thus, it is unlikely that embrittlement of steels by a liquid is responsible for the phenomenon of lead embrittlement.

It has been suggested [see Hirschhorn: Journal of Metals, 40-43 (October 1970)] that the hot workability of leaded brass can be improved by the addition of mischmetal to the brass to thereby reduce losses due to hot tearing. As pointed out in the aforementioned article, mischmetal (MM) is added to such brasses to neutralize the harmful effects of lead by reaction with the lead to form MMPb a high melting intermetallic compound which thus serves to prevent liquifaction 0f the lead at temperatures above 327 C. (610 F.) resulting in intergranular tears. Therefore, the effectiveness of mischmetal as an additive to brasses is apparently a liquid metal phenomenon.

It is accordingly an object of the present invention to provide a new and improved method for the elimination of lead embrittlement in steel.

It is a more specific object of this invention to provide a new and improved method for the treatment of leaded steels to provide a steel having significantly improved mechanical properties at high temperatures and having good machinability.

The concepts of the present invention reside in the discovery that the addition of one or more rare earth metals to a leaded steel provides a steel having significantly improved mechanical properties at elevated temperatures. The theory underlying the effectiveness of rare earth metal additives in the present invention is not understood at the present time. While, as indicated above,

the effectiveness of mischmetal in neutralizing the effects of lead in brass is through the formation of a lead-rare earth containing intermetallic compound to prevent intergranular tears, the presence of lead in steel has been found to result in embrittlement at temperatures well below the melting point of lead, and it can therefore be concluded that the effects of rare earth metals in steel are substantially different from the effects of rare earths in brass or the like.

As used herein, the term rare earth meta is intended to refer to and include rare earth elements having an atomic number of 58 or higher as Well as compounds thereof which liberate one or more rare earth metals when combined with steel. Representative of such metals include cerium, neodymium, samarium, praseodymium, gadolium, etc. Due to the fact that many of the rare earth metals are not readily available, and consequently, very expensive, it is frequently preferred to make use of mischmetal, a well known and commercially available mixture of the rare earth metals.

The amount of rare earth metal added to the steel depends somewhat on the amount of lead in the steel. In general, leaded steels contain up to about 0.35% by weight of lead, and thus use can be made of sufficient quantities of one or more rare earth metals to provide 0.01 to 0.5% by weight, and preferably 0.02 to 0.2% by weight rare earth metal in the steel.

The concepts of the present invention are applicable to any of the steels, including both high and low carbon steels and low, high or no alloy steels containing lead as an additive. In addition, the concepts of the present invention may also be applied to steels containing no lead or trace amounts of lead as an impurity to insure that any lead, if present, will be neutralized and not cause failure during the use of such steel in elevated temperature applications, and particularly in applications in which the steel is subjected to stresses at temperature within the range of 400 to 900 F.

The rare earth component may be added to the steel in any convenient manner, either before or after addition of the lead. For the sake of simplicity, it is generally preferred to add the rare earth component to a heat of the steel when the lead is added. After addition of the rare earth metal or metals, the heat can be processed in a conventional manner to provide a steel ready for machining having significantly improved mechanical properties at high temperatures, and particularly at temperatures of 400 to 900 F.

Having described the basic concepts of the invention, reference is now made to the following examples which are provided by way of illustration, and not of limitation of the practice of the invention.

EXAMPLE 1 Additions (percent by weight) Heat. A1 Pb Mischmetal A 0. 07 1. 40 B 0. 07 I. 40 0. 50

The heats are then processed in a conventional manner, and the resulting steels are forged into bars having the following compositions:

Chemical composition Heat A (percent by weight) The resulting bars are then heat treated to provide each with substantially the same hardness at a level at which embrittlement is most severe. For this purpose, the bars from Heat A are heated at 975 F. and the bars from Heat B at 900 F.

The bars are then tested at elevated temperatures to determine the embrittlement of the steels, and the results of these tests are shown in FIG. 2 of the drawing. As can be seen in this figure, the bars from Heat A containing no rare earths exhibit severe embrittlement at temperatures from 400 to 900 F., particularly in the middle of that temperature range. However, as can also be seen from this figure, the bars from Heat B containing the rare earths exhibit a significantly improved reduction in area and consequently significantly improved ductility, over the same temperature range.

EXAMPLE 2 The procedure of Example 1 is again repeated using cerium alone with a heat having substantially the same composition of the heats employed in Example 1. Comparable results are obtained.

It will be apparent from the foregoing that we have provided a new and improved method for substantially eliminating embrittlement of steel. The present invention provides a needed solution to the problems of failures of leaded steels subjected to stresses at high temperatures which have been experienced in increasing frequency in recent years.

It will be understood that various changes and modifications can be made in the details of procedure, operation and use without departing from the spirit of the invention, especially as defined within the scope of the following claims.

We claim:

1. A method for the manufacture of articles from steel comprising the steps of adding to molten steel containing lead in an amount sufiicient to cause embrittlement of the steel at least one rare earth metal in an amount sufiicient to eliminate lead embrittle'ment of the steel, forming the resulting steel into an article and subjecting the article to stress at a temperature within the range of 400 to 900 C. whereby the article exhibits improved mechanical properties at temperatures within this range.

2. A method as defined in claim 1 wherein the leaded steel contains up to about 0.35% by weight lead.

3. A method as defined in claim 1 wherein the rare earth metal is added in an amount sufficient to provide a steel containing 0.01 to 0.5% by weight of the rare earth metal.

4. A method as defined in claim 1 wherein the rare earth metal is a mixture of rare earth metal.

5. A method as defined in claim 4 wherein the mixture of rare earth metals is mischmetal.

6. A method for the manufacture of articles from steel comprising the steps of adding to steel containing lead in an amount sufiicient to cause embrittlement of the steel at least one rare earth element to eliminate lead embrittlement of the steel and forming the resulting steel to form an article having improved mechanical properties at elevated temperatures within the range of 400 to 900 C.

7. A method as defined in claim 6 wherein the steel is 3. References Cited leaded steel containing up to about 0.35% by wei ht lead.

8. A method as defined in claim 6 wherein lhe rare UNITED STATE S PATENTS earth metal is added in an amount sufiicient to provide 3,313,620 4/1967 corradml 75129 X g j fi gg f to 05% by Weght 0f the rare 5 L. DEWAYNE RUTLEDGE, Primary Examiner 9. A method as defined in claim 6 wherein the rare I. E. LEGRU, Assistant Examiner earth metal is a mixture of rare earth metals.

10. A method as defined in claim 9 wherein the mix- US. Cl. X.R. ture of rare earth metals is mischmetal. 10 75123 E, 123 F 11. A method as defined in claim 6 wherein the rare earth metal is added to a heat of the steel and the heat is formed into a semi-finish part suitable for machining. 

